In a membrane filtration operation, periodically cleaning the membrane by liquid or gas backwash is essential to keep a longer membrane operation time without the need for a chemical cleaning stage. However, during each backwash, a certain amount of liquid waste is produced, which reduces the feed liquid recovery and increases the requirements on post treatment of backwash is waste. The liquid or gas backwash is also supplemented by periodic cleaning of the membranes using a chemical cleaning agent. This process again produces liquid waste which must be further treated or disposed of in an environmentally safe manner. It is thus desirable in any filtration operation to minimise the volume of waste liquid produced during the backwash and chemical cleaning phases of operation in order to reduce costs of operation and any environmental impact.